Process and device for the distillative purification of metals, especially of zinc

ABSTRACT

Metal, especially zinc, is purified by distillation of the metal in the fused state in a packed column. The fused metal is introduced at the top of the packing so that a coherent stream of the metal flows through a considerable height of the packing. Heat is supplied to the packing to provide a temperature gradient across the packing so that the temperature of the vapor remote from the coherent stream is relatively high with respect to the temperature of the coherent stream.

United States Patent Lowicki [54] PROCESS AND DEVICE FOR THE DISTILLATIVE PURIFICATION OF METALS, ESPECIALLY OF ZINC Norbert Lowicki, Germany Grillo Werke Aktiengesellschaft, Duisburg- Hamborn, Germany Filed: July 19,1968

Appl. No.: 746,191

Inventor: Duisburg-Ham born,

Assignee:

[30] Foreign Application Priority Data July 20, 1967 Germany ..P 15 83 880.9

US. Cl ..75/63, 75/88, 203/100 Int. Cl ..C22b 9/02, C22b 19/14, BOld 3/28 Field of Search ..75/63, 88, 86; 202/127, 225,

[56] References Cited UNITED STATES PATENTS 1,086,452 10/1914 Golodetz ..202/158 X [451 Jan. 25, 1972 1,941,569 1/1934 McCraven ...75/88 X 2,183,535 12/1939 Betterton et al. ...75/88 X 2,433,615 12/1947 Mahler ..75/63 X 2,674,609 4/1954 Beal et a1. ..202/158 X 2,915,883 12/1959 Van Der Stel'... ..202/153 X 3,325,376 6/1967 Eckert ..202/l58 Primary Examiner-Henry W. Tarring, 11 Attorney-Burgess,- Dinklage & Sprung [57] ABSTRACT Metal, especially zinc, is purified by distillation of the metal in the fused state in a packed column. The fused metal is introduced at the top of the packing so that a coherent stream of the metal flows through a considerable height of the packing. Heat is supplied to the packing to provide a temperature gradient across the packing so that the temperature of the vapor remote from the coherent stream is relatively high with respect to the temperature of the coherent stream.

22 Claims, 1 Drawing Figure PATENIED #:4251972 INVENTOR NDRBEKT LowiLK 1 r 1 1 I I I 1 1 r 1 PROCESS AND DEVICE FOR THE DISTILLATIVE PURIFICATION OF METALS, ESPECIALLY OF ZINC The invention relates to a process for the distillative purification of metals, especially from their mixtures or alloys with less volatile metals. In particular, the invention relates to the distillative production of pure zinc from its alloys with, for example, lead, iron, aluminum and/or copper, for example for the manufacture of zinc white and pigment zinc dust.

It is known to use externally heated column for refining zinc by means of distillation. Herein the zinc is first separated by distillation from its less volatile accompanying metals. The latter are removed at the base of the first column while the zinc, and where appropriate its even more volatile accompanying metal cadmium, are together condensed at the head. These two metals are then separated in a second distillation in which the cadmium is withdrawn as the head product and the pure zinc as the sump product.

The distillation columns are equipped with dishlike plates through which liquid zinc is passed in countercurrent to the stream of metal vapor. The less volatile components of the metal vapor, above all the lead, condense on contact with the liquid metal surface and flow back into the sump. Such a rectification presumes that the less volatile accompanying metals which contaminate the zinc are liquid and/or soluble in the zinc at the process temperature and can be continuously removed in this form. In each case this zinc refining process requires a special process stage separate from the actual production of the zinc.

It has furthermore already been repeatedly proposed to remove less volatile metals accompanying the zinc, above all lead, already during the process of obtainingthe zinc. This is intended to avoid the renewed distillation of the zinc which has already once been condensed. Since zinc, when being obtained from the ores, is in any case produced in the vapor form it is obvious to separate off the higher-boiling components of the vapor mixture by a fractional condensation in a packed column. Proposals of this nature are for example contained in the following patent specifications: U.S. Pat. specification No. 1,4l7,953, and German Pat. specifications Nos. 529,431 and 674,338. These processes have however not found acceptance in industry.

The known New Jersey process for obtaining zinc also envisages a certain preliminary separation of the lead from the zinc vapors. U.S. Pat. specification Nos. 1,863,711, l,9l4,482, l,9l4,483 and 1,914,484 contain proposals as to how a preliminary condensation of the lead in the upper colder part of the fresh charge might be effected. As is known, however, this does not make it possible to obtain a practically lead free zinc.

Analogously to the distillative separation of the mixtures of substances by rectification using packed columns, considera tion has also already been given to pass zinc vapor through columns filled with lumps of refractory material or with coke and to return the condensed reflux into the retort in which the zinc is vaporized. The advantage of the more intense material exchange between the vapor phase and the liquid phase in the packed column, in comparison to the distillation column with dishes, is here obvious. Such proposals apply to the purificatioh or separation of zinc alloys, above all of those obtained on melting down zinc shot or also those which are for example produced as a byproduct, in the form of so-called hard zinc, on flame-galvanizing of iron articles. Thus Gennan Pat. specification No. 586,627 proposes refining hard zinc by means of an unheated insulated packed column. Since however a solid iron sponge remains on distillation of hard zinc, continuous distillative working-up of hard zinc in the manner described is not possible. The distillation retort must frequently be mechanically cleaned. The air which hereupon enters oxidizes the metal remaining in the packed column and the oxide products thus produced block the column.

ln order to circumvent these difficulties some proposals described distillation processes with electrothermal heating, as for example in German published specification No. 1,192,411. A coke column here serves as the heating resistance and the current is supplied by an electrode which reaches into the column from above. The crucible at the lower end serves as the counterelectrode. The heat required for the vaporization of the zinc is here produced by the electric current mainly in the thin layers of zinc metal on the surface of the pieces of coke. It is therefore stated to be essential to the process that the metal to be refined is distributed very uniform from four charging orifices, over the entire surface of the coke column. Hard zinc to be processed is comminuted, mixed with clay, briquetted and introduced together with the coke. The remaining iron is removed together with the clay as the charge migrates downwards and is then separated from the coke.

An electrothermal process with a similar procedure is described in German Pat. specification No. l,l 18,475. Here again a packed column of coke, SiC or other electrically conducting materials is heated to the distillation temperature of the zinc. The zinc is then removed in the vapor form and condensed to give the metal. The aim of this process is however not a distillative purification of zinc alloys but the production of the metal from zinc dusts produced as byproducts in the course of a primary process.

U.S. Pat. specification No. 2,939,783 describes a further process variant for the distillative refining of zinc shot, pressure-casting alloys and the like. This is effected in two stages, and in the first stage the vaporization of the zinc is only taken to the point that the metal vapor, after passing through a column with guide walls arranged zigzag fashion, is completely pure. The sump alloy, in which the impurities have become enriched, is then further distilled in the second stage and these vapors are condensed via a packed column. The prepurified zinc alloy thereby produced is returned to the first distillation stage. The mode of action of the packed column is not explained further.

In these proposals from the state of the art, using electrical resistance heating of the entire packed column, a very uniform temperature results over the length and cross section of the column. The heating electrodes passed vertically through the column also have the same effect. It is an essential element of the invention to show that this old concept has a negative infiuence on the conduct of the process for the distillative purification of the metal.

The description of the invention which follows shows that heating the packed column by electrical generation of heat in the metal film trickling down over the packing is even more harmful to the purification effect. This is because it makes it impossible to remove the amounts of heat liberated on condensation of the impurities which are to be removed.

The invention is based on the fundamental new recognition that in order to have effective rectification of a mixture of materials it is not only necessary to provide adequate material exchange between the vapor phase and liquid phase and preferably at the same time to set up a temperature gradient in the direction of the stream of vapor, that is to say to provide a vertical temperature gradient, but that beyond this the action of the rectification unit must be decisively influenced by the fact that additionally a horizontal temperature gradient, that is to say a radial temperature gradient at right angles to the direction of flow of the gas phase and vapor phase, is provided within the rectification unit. It has been found that the operation of a rectification unit having such a radial temperature gradient leads to a great increase in the separating action of the rectification unit and that such conditions can in particular be realized in a technically simple manner in the distillative treatment of metal, so that distillations with a hitherto unknown degree of effectiveness can here be carried out. The process according to the invention which is based on these recognitions opens up the possibility of purifying distillable metals, of which zinc or cadmium should be especially mentioned, in a simple and continuous procedure. The new process is at the same time particularly suitable for workingup zinc alloys such as zinc shot or other zinc scrap containing impurities boiling higher than zinc.

Accordingly the subject of the invention is a process for the distillative purification of metals and especially for obtaining zinc from its alloys with the conjoint use of fractionating columns, characterized in that the fused crude metal which is to be purified is introduced in the form of at least a coherent stream of liquid from above into an externally heated vertical packed column in such amounts that at least a coherent stream of fused crude metal flows through at least a considerable part of the length of the packed column, while the vaporized metal is passed upwards through the packed column in countercurrent thereto and is withdrawn at the top. In a further development of the invention the vapor stream withdrawn at the head of the column can if desired to be freed of metal droplets which have been carried along.

The essential feature of the invention thus resides in a fractionation stable. This fractionation stage takes place in a packed column into which the fused crude metal introduced is at the top as a coherent stream of liquid. The amount of liquid crude metal introduced is at the same time metered in such a way, especially matching it to the heating-up temperature in the column and the amount of vapor passing upwards through the column, that a coherent stream of the fused metal flows downwards through the packed column like a rivulet or brook. The formation of such a continuous rivulet passing downwards right through the packing is favored by the characteristics of the liquid phase, especially probably by the high specific gravity of the fused metal and by its surface forces. In any case it hasbeen found that a condition is established which is not directly comparable with the operation of the usual packed column, for example in the distillation of organic liquids. While in the usual operation of the packed column as uniform as possible a spreading of the liquid column reflux over the entire cross section of the column is desired and thus a uniform column temperature, viewed horizontally, is established, the application of a continuous stream of fused metals of the kind here under discussion onto a conventional packing leads to the primary formation of a continuous, or only comparatively slightly branched, rivulet of the liquid phase through the column bed. Admittedly a part of the liquid is here also branched off laterally and thus distributed itself over the entire packed column, but nevertheless a downward-flowing stream of the liquid phase obviously forms from the rivulet which flows down through the packing, preferably in a continuous form. The hot metal vapor flows upwards alongside this rivulet. An exchange between the liquid and vapor phase is possible as a result of the adequate wetting of the packing by the liquid phase and to this extent the operation of the packed column in the process of the invention also corresponds to the conventional use of such a column. Differing therefrom-and this is obviously one of the essential central points of the new process -a radial temperature gradient is however simultaneously established in a horizontal cross section through the packed column, that is to say the horizontal cross section of the packed column does not show the same temperatures at all points during operation.

The essentially continuous downward-flowing rivulet of the fused metal, which has actually not yet been heated to vaporization temperatures, is comparatively colder than the ascending metal vapor and than the externally heated wall of the packed column. At the same time it is important that this comparatively cool downward-flowing rivulet of the continuous stream of metal is more or less fixed in its position in the cross section of the packed column. If for example the stream of fused crude metal is charged centrally into the packed column, then the rivulet also flows downwards approximately centrally through the vertical packed column. A quasi'static condition as regards the radial temperature gradient in the column can in this way result, and this leads to the metal vapor passing upward through the column being constantly exposed to the influence of this virtual fixed radial temperature gradient. It will further become clear that the desired fractionating action of the column is hereby surprisingly increased relative to the usual operation with a horizontally uniform temperature or only a statistically distributed and constantly changing temperature gradient.

In addition to the radial temperature gradient, a temperature drop in the direction of the stream of vapor, that is to say a vertical temperature gradient, also establishes itself in the packed column. This occurs automatically as a result of the fact that the colder liquid phase is introduced at the head of the column. In this way the stream of vapor can be cooled down to a temperature below the evaporation temperature of the-less volatile components which are to be separated off. The most volatile components, that is to say for example zinc in the particular case, leaves the packed column at the head.

and is then condensed.

In the process of the invention, the temperature gradient at right angles to the direction of flow of the liquid and vapor phase is appropriately established to the extent that the temperature of the colder part of the column which extends vertically through the packed column, that is to say the downwardflowing rivulet of the fused initial material-lies below the boiling point of the most volatile impurity which is to be removed. Preferably a temperature difference which is not too small is at the same time chosen. In this way the deposition of all higher-boiling components of the alloy from the metal vapor which is to be purified is achieved to an increased extent. This is because the radial temperature gradient has the effect that the partial pressure of the individual components drops considerably at right angles to their direction of flow, from the hot outer wall to the relatively cold core zone of the column. This considerably intensifies the migration of the vapor parts of the higher-boiling impurities towards the center of the column. There these impurities meet the downwardflowing stream of metal and are thereby flushed downwards through the column. This demonstrates a further action, essential to the invention, of the continuous stream of fused metal which is passed through the packed column. The stream of metal collects the undesired higher-boiling impurities and simultaneously washes them downwards in the column, that is to say in the direction in which they should be passed in accordance with their destination.

In carrying out the process according to the invention, radial temperature gradients in the horizontal cross section of at least C. are preferred. In the distillation of zinc, the radial temperature gradient can for example assume values of between l50| and 550 C.

The material exchange processes in a rectification column are accompanied by a considerable exchange of quantities of heat. These result from the quantities of heat consumed for the vaporization of the alloy components and the quantities of heat liberated on condensation of these. To this must be added the heats of mixing between the individual alloy components, which are also consumed or liberated. Investigations carried out on conventional packed rectification columns have shown that zones which are statistically distributed and constantly changing are formed within the column which must be described as undercooled or strongly overheated relative to the vapor of the easily volatile component which is to be rectified. It is clear that this makes a controlled separation of the less volatile components considerably more difficult. The depletion of this less volatile component in the vapor occurs in accordance with an adventiu'ously arising statistical depletion over the height of the column.

As a result of the establishment of the radial temperature gradient in accordance with the invention, a definite temperature gradient directed towards the center of the packedcolumn develops between the heated outer wall and the preferably centrally introduced stream of metal, and this temperature gradient forms even if only such an amount of metal is charged into the column that on passing through the entire heated column height it can be evaporated approximately completely. Now in order to reinforce this radially directed temperature gradient, it is preferred, in a further embodiment of the invention, not only to introduce such an amount of the crude metal as corresponds to the distillation capacity of the column in accordance with the possible heat transfer, but to introduce more and indeed optionally also considerably more. In this embodiment of the invention a larger quantity of fused crude metal is thus introduced at the head of the packed column than is withdrawn in the vapor phase from the head of the column. Herein the equilibrium ratios between the liquid phase and the vapor phase may, in a preferred embodiment, be so adjusted that to 90 percent by weight, preferably to 70 percent by weight, of the desired metal from the crude metal charged into the column are evaporated and withdrawn at the head of the column. The excess of crude metal introduced serves to form the downward-flowing liquid cooler core of the rectification column and at the same time takes up the higher-boiling impurities which are to be separated out and leaves the packed column at the bottom together with these impurities. The less volatile alloy constituents which have become enriched in the sump product withdrawn at the base of the column can be in part separated from the sump product in a conventional manner, for example by refining. Fine refining is not necessary. In the continuous process it is merely necessary to ensure that no undesired enrichment in impurities takes place. This is because in that case the sump product which has been partially freed of impurities is preferably returned to the column head and here returned to the separation process, mixed with fresh fused crude metal. This circulation of the stream of metal makes it possible to remove the less volatile impurities of the alloy, which have been separated out during the material exchange in the column, from the rectification process in a controlled manner and to enrich them in the sump product in the desired manner.

The evaporation of the desired metal constituent from the stream of crude metal is, according to the invention, preferably effected within the packed column by adjusting the amount of heat introduced relative to the amount of the crude metal introduced, taking the equipment constants into ac count. Admittedly such evaporation of the metal within the packed column is not necessary. It is also possible to provide a separate evaporation unit at the base of the packed column and then to introduce the metal vapor into the packed column from below. As a rule however this requires greater expenditure on apparatus.

In carrying out the new process it proves advantageous effectively to divide the packed column approximately into at least 2 sections which are distinguished by differing heating. The lower section is adjusted to be hotter than the section arranged above it and serves as the evaporator zone. It is here that the major part of the metal vapor is thus produced from the crude metal stream. The cooler part arranged above it serves for the fractionation and is accordingly described as the fractionating zone. The task of this section of the column is to ensure that the desired exchange processes take place and these should be disturbed as little as possible by additionally evaporating metal. A far-reaching control of the effective throughput as well as of the purity of the product distilled off can be effected by controlling the heat input in the lower evaporator part and in the superimposed fraetionating part, of the column. Further influence of the throughput and purity can be achieved by adjusting the circulation ratio and/or by adjusting the amount of the constituent of the distillate, to be obtained in a pure form, which is evaporated from the crude metal. Such a far-reaching possibility of regulating the course and result of the distillation has hitherto not been available in any rectification process for metals.

The purified metal vapor produced at the column head on rectification can admittedly contain a not insignificant proportion of very fine liquid droplets which are mechanically carried along by the stream of vapor and in which there are still dissolved comparatively considerable quantities of the impurities. This again somewhat contaminates the inherently pure metal vapor and hence the distillation product. In the distillation processes for the purification of zinc or other easily volatile metals which have hitherto become known no proposals are made for effectively excluding such mechani cally entrained impurities. In a further characteristic of the invention this problem is overcome by additional process measures. According to the invention, the following procedure can be adopted for this purpose:

The metal vapor issuing at the head of the column is not passed to the cooling directly but is further passed through an additional process stage for separating off the mechanically entrained, droplet impurities. Here, conventional methods for separating off a very finely divided liquid phase from a vapor plane can be chosen. In a particular embodiment of the new process, however, particularly simple means are here used which can effortlessly be joined onto the process means hitherto described. In this embodiment the stream of vapor, after leaving the packed column, is passed from below into a further packing layer. Here the separation of the metal droplets is effected by a filtration effect. At the same time the temperature of this part of the column can be so adjusted by appropriate regulation of the external heating that it is approximately the same as the temperature of the stream of vapor. In this case the packing layer exclusively acts as a kind of filter which very largely retains the liquid particles. In order to intensify the separation of the liquid particles the temperature of the packing layer can however also be chosen to be somewhat lower.

In that case a restricted deliberately produced condensation of a part of the ascending metal vapor results in an increase in the metal droplets and hence in an additional washing effect on the metal vapor, that is to say for example the vaponform zinc. This effect can also be further increased by trickling the pure liquid metal, that is to say for example liquid zinc, onto this packing layer from above. In a particular embodiment of the new process almost equally good purification of the zinc vapor from mechanically entrained impurities is however achieved if the temperature of the packing layer which has been described is chosen to be higher than the temperature of the zinc vapor which leaves the packing column at the top. No trickling onto the packing layer used for purification is then employed; rather, an overheating of the zinc vapor, and hence an increase in volume of the pro-purified metal vapor is effected in the layer. This overheating of the zinc vapor is however only taken to a temperature which still lies below the boiling point of the impurities which are carried along. These measures result in vaporization of the mechanically entrained liquid zinc metal droplets and of the less volatile impurities as a result of the increased turbulence of the flowing vapor in the layer of poured-out material on the surface of the packing. The practically pure zinc metal vapor can then be withdrawn at the top at the packing layer.

Now in order also to ensure reliable separation of the higher-boiling impurities from the packing layer in continuous and extended operation, a particularly preferred embodiment of the invention provides that a radial temperature gradient is also formed in this packing layer which is used for the purification of the vapor. According to the invention this can be achieved in a simple manner by placing the packing layer onto which there is no trickling directly on top of the packed column charged wit the crude metal and creating a colder zone extending from the top through to the bottom in the interior of the packing layer, such as for example centrally. This formation of the colder zone is very simply achieved by passing a tube through the packing layer from top to bottom and passing the stream of crude metal to the head of the packed column through this tube. In this embodiment of the invention the rectification of the metal thus takes place in a two-part or three-part rectification column which is characterized in that a feed tube for the fused crude metal alloy reaches into the layer of poured-outmaterial (i.e., packing material) from above. This feed tube is preferably arranged to be adjustable in height and it is appropriate to adjust the depth of penetration into the layer of poured-out material to a range of about one-third to one-eighth of the entire height of the poured-out material layer. This column of poured-out material is then divided into 3 zones by differential heating. The

lowest zone serves for the vaporization of the metal and is as a rule the hottest. The fractionation zone, in which the material exchange processes occur, is formed above this with gentler heating. It finishes, in the upper direction, at the mouth of the feed tube for the crude metal alloy which is to be purified and is followed there by the uppermost zone of the poured-out material column which is again adjusted to be hotter than the fractionating zone below it. This uppermost zone serves for the separation of metal vapor and liquid impurities which has been described. The feed tube fed with the stream of molten metal effects a sufficiently large temperature gradient from the heated wall to the center to allow the gradually accumulating droplets of less volatile impurities to trickle down on the outer wall of the feed tube. These combine with the stream of metal which issues into the layer of poured-out material lower down and are carried with this into the sump product.

In order to avoid a vaporization of the metal introduced occurring within the feed tube the latter can be protected against excessive heat transfer by a heat-resistant and corrosion-resistant insulating layer to the point that the temperature within the feed tube is reliably below the boiling point of the most volatile alloy constituent. Nevertheless the alloy is slowly heated in this feed tube.

It is even possible to fuse suitably lumped solid blocks of the alloy to be purified at the feed tube. This cushion of liquid crude metal then reliably seals the column content off against the atmosphere so that special devices are not needed. The hydrostatic pressure of the liquid metal column in the feed tube prevents metal vapor issuing outwards and seals off the distillation system reliably against atmospheric oxygen. The amount of metal flowing into the column can be crudely regulated by the diameter of the feed tube and sensitively regulated by the height of the metal column therein.

In the normal case it can be appropriate to allow the feed tube to dip into the column to the extent of about one-third so that in the triple division into an evaporating zone, a fractionating zone and an upper purification zone approximately one-third of the total column length is in each case available for one of these zones. In special cases it can however be desirable to enlarge the upper purification zone by allowing the feed tube to penetrate more deeply, for example up to two-thirds of the column length. The evaporation and fractionation zones then form below this. In a further modification it is however also possible to undercool the upper purification zone somewhat, at least partially-adjoining the fractionation zone-by that here a reflux of the metal to be purified in the liquid phase is set up. In this way a fractionating effect is also achieved here. Furthermore the entire fractionating zone can in special cases also be run in such a way that here a partial liquidification of the metal vapor and a corresponding reflux of the liquid phase results in the fractionating effect.

The possibility of changing the depth of penetration of the feed tube into the column and hence of adapting it to the particular composition of the crude metal which is to be purified leads to a further possible variation in the new process. This is because the depth of penetration of the feed tube can be adapted to the nature and amount of the contents of impurities in the starting alloy. If the crude metal predominantly contains elements which are particularly extensively entrained mechanically in zinc droplets (for example aluminum and copper) the depth of penetration of the feed tube and hence the height of the overheating zone or filter zone should be made large. if on the other hand elements are predominantly present which have to be separated out by condensation or solution in the reflux stream (for example lead or iron) the depth of penetration can be made less and the fractionating zone accordingly larger. The heating zones along the column are appropriately regulated and set to the particular temperature level required.

Accordingly, the following possibilities are, according to the invention, available for adjusting the desired degree of purity of the distillate and/or for adapting the process to the nature and amount of the impurities in the crude metal as well as for adaptation to the desired enrichment in foreign metals in the sump product:

1. Regulation of the amount of metal introduced into the feedpipe by controlling the metal influx by means of the feedpipe diameter as well as the height of the metal surface set up in the feedpipe.

2. Changing the length of the temperature zones along the fractionating column and hence choosing the height of the evaporation zone and/or fractionating zone.

3. Deciding the particular heat input supply and hence choosing the proportion of metal to be evaporated in relation to the total metal passed through.

4. Changing the depth of penetration of the feedpipe and hence varying the height of the uppermost layer of poured-out material (i.e., packing), onto which there is no trickling, in order to separate out mechanically entrained impurities.

5. Height of the total layer of poured-out material.

By appropriately adjusting these possibilities relative to one another and taking into account the effects which are thereby set up, that is to say especially also the radial temperature gradient, it is possible according to the invention to achieve very high purity of the distillation product with the aid of comparatively extraordinarily small distillation devices.

The circumstance that on evaporation in the packed column a deposition of metallic impurities on the packing is very largely prevented by their solution in the trickling-down metal stream can furthermore be advantageous for carrying out the new process. For example, iron can in this way be dissolved, or kept i solution by a certain aluminum content in the trickling-down zinc. Solid deposits, especially of an oxide nature, or mechanical impurities of the metal introduced only represent a minor part. If they are deposited on the packing material, then the latter is continuously or periodically withdrawn from the column, freed from these impurities and again reintroduced at the head of the column.

The head of the rectification column preferably remains empty, and thereby a streamlining space for the metal vapors is produced and dust from the packing which may have been entrained can be separated out. This is under certain circumstances of importance in the manufacture of pigment zinc dust from the zinc vapor or in the case of high exit speeds of the zinc vapor for special methods of manufacture of zinc white.

The zinc vapor exit speed from the column can be regulated to meet requirements, apart from doing so by the distillation speed, also by the choice of the diameter of the vapor exit pipe as well as by optional arrangement of one or more of these in the lid of the column.

At the same time the process according to the invention is not only distinguished by the fact that it permits a particularly effective and simple purification; it is now possible to solve, in terms of distillation, problems which could not be solved according to the prior technique of distillation of metals, espe cially easily volatile metals such as zinc. Thus it is known that the distillative purification of zinc in dish columns only produces high grade zinc from foundry red zinc which has been prepurified by refining. The amount of the impurities of the zinc which are first runs before this rectification is limited and should if possible not exceed l.5 percent in the case of the main impurity, lead. The contents of other impurities are then significantly lower, for example 0.2 to 0.5 percent in the case of Fe and 0.005 percent and below in the case of Cu and Al. Against this there is for practical purposes no limit to the content of higher-boiling impurities for the process according to the invention. For example, in the distillation of zinc the level of the lead and/or iron may be 6 percent and copper and/or aluminum 10 percent and even above.

The invention furthermore relates to an apparatus for carrying out the new process. In order to describe this, reference is made to the attached FIGURE which shows a schematic designed drawing of the part of the column which is essential for the distillation.

The upright, preferably vertical, distillation column or retort 7, consisting for example of silicon carbide, is filled with a packing in small pieces, having the usual particle size in relation to the column diameter, for example with coke in small pieces, 17. The feed device for recharging the packing reaches into the retort to an extent such that the conically widened head 6 remains empty and can serve as a streamlining space for the metal vapors. These are removed through one or more exit pipes 3 into the condenser 16 or into a combustion chamber for the manufacture of zinc white, not shown in the drawing. The widened-out column head 6 is closed by a lid through which, inter alia, the feedpipe 2 for the crude metal which is to be purified projects downwards into the interior of the column. This feedpipe 2 can be adjusted in height in a conventional manner which is not represented in the drawing. it is encased in a refractory and corrosion-resistant insulating material 4, at least over the part which projects into the column. It is continuously fed with crude metal via the supply chute 1. This crude metal can be prefused by means of waste heat, in a fusing kettle which is not represented in the drawing and which is arranged in the stream of waste gas.

The metal which trickles down the packed column and is enriched in the higher-boiling impurities which have separated out is withdrawn continuously or batchwise through the syphon 8/9, in a manner corresponding to the amount introduced and the distillation conditions. In order to renew the packing, the latter can be removed through the orifice 10 as required and be reintroduced through 5 after having been cleaned.

The column is externally heated by means of several burners ll, 12 and 13 arranged vertically above one another or arranged staggered. In practice it is appropriate to use more than the 3 burners represented here. These burners are built into a jacket of refractory material. The latter can be constructed in such a way that not only one but two or more distillation columns can be simultaneously operated in parallel. This permits more economical utilization of heat as well as a simplification of the feed and of the other servicing processes. The prefusing of, for example, zinc shot can then take place centrally at one point by means of utilization of waste heat. The packed column can also be constructed as a long rectangle, viewed in a horizontal section, and in that case several inlet pipes 2 are arranged at a certain distance from one another and preferably centrally between the two nearest external walls.

The 3 zones of the column are schematically represented by the 3 burners 11, 12 and 13. The burner 13 covers the hottest evaporation zone, the burner 12 heats the fractionating zone which is set to the lower temperature and which ends at the mouth of the inlet pipe 2, and finally the burner 11 heats the layer of poured-out material (i.e., packing), onto which there is no trickling, above the fractionating zone to the desired temperatures.

EXAMPLE 1 A hard zinc of high lead content, containing 2.6% Pb and 3.3% Fe, is refined in the distillation column described above. The depth of penetration of the feed pipe was set to oneseventh of the total height of poured-out material in the column and the fractionating part was lengthened to half the height of the.column. Coke in small pieces, of particle sizes between 12 and mm., served as the packing.

The temperatures in the evaporator part of the column were set to 1,230l,250 and those in the fractionating part to 1,100P-1 ,l20, in each case measured at the inner wall of the column. The temperature in the layer of poured-out material (i.e., packing) above the feedpipe outlet, onto which no trickling occurred, lay in the same range as that in the fractionating part since overheating of the metal vapor was not necessary. The metal reflux was regulated to 50 60 percent relative to fresh alloy. The distillation product contained 0.008% Pb and 0.0003% Fe and corresponded to a normal quality of high grade zinc.

EXAMPLE 2 A zinc pressure casting alloy shot contaminated with brass, containing 5.2% A1, 3.8% Cu, 0.5% Pb and 0.3% Fe was converted into pigment zinc dust by means of a rectification column. Because of the high first-run of impurities, which are easily mechanically entrained, the depth of penetration of the feedpipe was adjusted to one-third of the total height of poured-out material (i.e., packing). The temperatures of the individual burner planes were regulated in such a way that the evaporator and the fractionating part also each amounted to one-third of the column height. The temperatures in these zones were as in example 1 and the temperature in the over heating part was set to l,150-l,200. The metal reflux was 25-30 percent.

The pigment zinc dust obtained contained less than 0.001% Cu, 0.015% A1, 0.1% Pb and 0.001% Fe.

EXAMPLE 3 A mixture of hard zinc, pressure-casting shot and other shot was worked up into zinc white by rectification and subsequent combustion. The starting alloy contained 2.2% Pb, 3.7% A1, 2.3% Fe and 1.4% Cu. The depth of penetration of the feedpipe as well as the heights of the evaporator and fractionating part were set as in example 2. The temperatures in these parts were retained and the overheating temperature was raised to l,250-1,280. The metal reflux was increased to S060 percent. The zinc white obtained contained: 0.02% PbO, 0.0017% A1 0 Cu and Fe each less than 0.0005 percent. It thus met the quality requirements of the rubber industry.

EXAMPLE 4 A hard zinc contaminated with brass, which was unusable for the usual manufacture of zinc white, was worked up into rolling zinc with the aid of the rectification column according to the invention.

The starting alloy contained 1.1% Pb, 6.2% Fe, 1.3% Cu and 0.8% A1. The distillation conditions were set as described in example 1 but the depth of penetration of the feedpipe was increased to one-fourth of the column of poured-out material (i.e., packing). The process was carried out without reflux and the amount of metal run in 'was set for maximum distilling-out of the zinc content. The distilled-off zinc contained 0.104% Pb, 0.003% Cu, 0.025% A1 and 0.0005% Fe.

The residue, which was discontinuously withdrawn at the base of the column, contained only 12.2% Zn. The impurities had become enriched therein, to reach the following contents: Pb 9.3%, Fe 56.6%, Cu 16.2%, Al 6.4%.

A part of these had deposited on the packing. On completion of this distillation period the packing was removed, cleaned and reintroduced.

EXAMPLE 5 An alloy containing 75.5% Zn, 9.1% A1, 9.0% Cu, 3.4% Pb and 1.4 Fe was worked up into rolling zinc. The distillation conditions and the depth of penetration of the feedpipe were set as described in example 3 but the process was carried out without reflux.

The sump alloy, which was withdrawn discontinuously, contained: 7.2% Zn, 42.2% A1, 30.6% Cu, 12.4% Pb and 5.9% Fe.

The zinc distilled off corresponded to a good quality of rolling zinc, as in example 4. it contained: 0.37% Pb, 0.032% A1 0.0027% Cu and 0.0008% Fe.

After the distillation, the packing was freed of metal droplets and oxidic impurities as well as other mechanical impurities by sieving and was reused.

Iclaim:

1. Process for purification of metal by distillation of the metal in the fused state wherein a vapor distilled from the metal and fused metal are passed countercurrently through a vertically disposed body of columnpacking for the distillation characterized in that the fluxed metal is introduced into an upper level of the packing spaced from the sides thereof in the form of a coherent stream of the fused metal in such an amount that said stream flows through at least a considerable height of the packing, and supplying heat to the packing to provide a temperature gradient radially across the packing so that the temperature of the vapor remote from the coherent stream is relatively high with respect to the temperature of the coherent stream.

2. Process according to claim 1, wherein the supplying of heat to the packing provides a vertical temperature gradient in the packing, the temperature at the bottom of the packing being relatively high with respect to the temperature at the top of the packing.

3. Process according to claim I, wherein some of the fused metal branches off laterally from said stream over said height of packing and distributes itself over the packing surrounding said stream, and said vapor passes through said surrounding packing contacting the molten metal distributed thereon.

4. Process according to claim 1, said metal being impure zinc.

5. Process according to claim 4, wherein the amount of zinc in the fused metal is introduced into said upper level of the packing exceeds the zinc product of the distillation process.

6. Process according to claim 5, wherein the zinc distillation product is about 20-90 percent by weight of the fused metal introduced into the column.

7. Process according to claim 5, wherein a sump product containing zinc is collected from a lower level of the packing, the sump product is refined to remove part of the impurities thereof and the resulting refined portion is combined with the fused metal subjected to said purification.

8. Process according to claim 4, wherein the supplying of heat to the packing provides a vertical temperature gradient in the packing, the temperature at the bottom of the packing being relatively high with respect to the temperature at the top of the packing.

9. Process according to claim 4, wherein some of the fused metal branches off laterally from said stream over said height of packing and distributes itself over the packing surrounding said stream, and said vapor passes through said surrounding packing contacting the molten metal distributed thereon.

10. Process according to claim 4, wherein said vapor distilled from the fused metal is distilled from the fused metal in said body of column packing.

11. Process according to claim 10, wherein said body of column packing includes an upper portion defining a fractionation zone, and a lower portion defining an evaporation zone and maintaining the lower portion at higher temperature than the upper portion.

12. Process according to claim 4, the radial temperature gradient between said stream and the wall of the column containing the packing, being at least 1008 C.

13. Process according to claim 12, the radial temperature gradient between said stream and the wall of the column containing the packing, being about l50.-550 C.

14. Process according to claim 4, wherein vapor from said distillation is passed through heated column packing for removal of entrained metal.

15. Process according to claim 14, wherein the column packing for removal of entrained metal is superimposed on the packing in which the distillation is performed, and wherein fused metal subjected to said purification is introduced thereinto by a pipe passing through the packing provided for removal of entrained metal.

16. Process according to claim 15, wherein the packing provided for removal of entrained metal is maintained at a higher temperature than the vapor from the distillation.

17. Process according to claim 15, wherein the packing provided for removal of entrained metal is maintained at a lower temperature than the vapor from the distillation.

18. Process according to claim I, wherein:

a. said vapor distilled from the fused metal is distilled from the fused metal in said bod of column packing, b. the supplying of heat to t e packing provides a vertical temperature gradient in the packing, the temperature at the bottom of the packing being relatively high with respect to the temperature at the top of the packing, and

c. some of the fused metal branches of? laterally from said stream over said height of packing and distributes itself over the packing surrounding said stream, and said vapor passes through said surrounding packing contacting the molten metal distributed thereon.

19. Process according to claim 18, said metal being impure ZmC.

20. Process according to claim 19, the radial temperature gradient between said stream and the wall of the column containing the packing, being at least C.

21. Process according to claim 19, wherein a sump product containing zinc is collected from a lower level of the packing, the sump product is refined to remove part of the impurities thereof and the resulting refined portion is combined with the fused metal subjected to said purification.

22. Process according to claim 21, the radial temperature gradient between said stream and the wall of the column containing the packing, being at least 100 C.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 637 367 Dated January; 25 19 72 Inventor(s) Norbert Lowicki It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 3, line 13, cancel "to".

C01. 3, line 16, "stable", should be --stage--. Col. 3, line 41, "distributed" should be --distributes--.

Col. 6, line 59, "wit" should be ---with--.

C01. 7, line 24, "lumped" should be --shaped-.

Col. 10, line 67, insert a comma after "A1".

Col. 11, line 1, "fluxed" should be -fused--.

Signed and sealed 6th 61 a bf March 1973: l I

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

2. Process according to claim 1, wherein the supplying of heat to the packing provides a vertical temperature gradient in the packing, the temperature at the bottom of the packing being relatively high with respect to the temperature at the top of the packing.
 3. Process according to claim 1, wherein some of the fused metal branches off laterally from said stream over said height of packing and distributes itself over the packing surrounding said stream, and said vapor passes through said surrounding packing contacting the molten metal distributed thereon.
 4. Process according to claim 1, said metal being impure zinc.
 5. Process according to claim 4, wherein the amount of zinc in the fused metal is introduced into said upper level of the packing exceeds the zinc product of the distillation process.
 6. Process according to claim 5, wherein the zinc distillation product is about 20-90 percent by weight of the fused metal introduced into the column.
 7. Process according to claim 5, wherein a sump product containing zinc is collected from a lower level of the packing, the sump product is refined to remove part of the impurities thereof and the resulting refined portion is combined with the fused metal subjected to said purification.
 8. Process according to claim 4, wherein the supplying of heat to the packing provides a vertical temperature gradient in the packing, the temperature at the bottom of the packing being relatively high with respect to the temperature at the top of the packing.
 9. Process according to claim 4, wherein some of the fused metal branches off laterally from said stream over said height of packing and distributes itself over the packing surrounding said stream, and said vapor passes through said surrounding packing contacting the molten metal distributed thereon.
 10. Process according to claim 4, wherein said vapor distilled from the fused metal is distilled from the fused metal in said body of column packing.
 11. Process according to claim 10, wherein said body of column packing includes an upper portion defining a fractionation zone, and a lower portion defining an evaporation zone and maintaining the lower portion at higher temperature than the upper portion.
 12. Process according to claim 4, the radial temperature gradient between said stream and the wall of the column containing the packing, being at least 100* C.
 13. Process according to claim 12, the radial temperature gradient between said stream and the wall of the column containing the packing, being about 150*-550* C.
 14. Process according to claim 4, wherein vapor from said distillation is passed through heated column packing for removal of entrained metal.
 15. Process according to claim 14, wherein the column packing for removal of entrained metal is superimposed on the packing in which the distillation is performed, and wherein fused metal subjected to said purification is introduced thereinto by a pipe passing through the packing provided for removal of entrained metal.
 16. Process according to claim 15, wherein the packing provided for removal of entrained metal is maintained at a higher temperature than the vapor from the distillation.
 17. Process according to claim 15, wherein the packing provided for removal of entrained metal is maintained At a lower temperature than the vapor from the distillation.
 18. Process according to claim 1, wherein: a. said vapor distilled from the fused metal is distilled from the fused metal in said body of column packing, b. the supplying of heat to the packing provides a vertical temperature gradient in the packing, the temperature at the bottom of the packing being relatively high with respect to the temperature at the top of the packing, and c. some of the fused metal branches off laterally from said stream over said height of packing and distributes itself over the packing surrounding said stream, and said vapor passes through said surrounding packing contacting the molten metal distributed thereon.
 19. Process according to claim 18, said metal being impure zinc.
 20. Process according to claim 19, the radial temperature gradient between said stream and the wall of the column containing the packing, being at least 100* C.
 21. Process according to claim 19, wherein a sump product containing zinc is collected from a lower level of the packing, the sump product is refined to remove part of the impurities thereof and the resulting refined portion is combined with the fused metal subjected to said purification.
 22. Process according to claim 21, the radial temperature gradient between said stream and the wall of the column containing the packing, being at least 100* C. 